Extraction control system for a turbogenerator set

ABSTRACT

A turbogenerator set having an extraction control system for maintaining a selected pressure at each steam extraction point regardless of the flow at individual extraction points and irrespective of whether the load on the generator is maintained at a selected load or whether the load is varied, as long as the load on the generator is greater than the power supplied by that portion of the extracted steam which passes through the turbine.

United States Patent Bryant 5] Apr. 3, 1973 [54] EXTRACTION CONTROL SYSTEM FOR A TURBOGENERATOR SET Primary ExaminerAlan Cohan Assistant Examiner-Allen M. Ost'rager 75 I t N. B I 1 men or ozro ryant Chester Pa AttorneyA. T. Stratton, F. P. Lyle and F. Cristiano, [73] Assignee: Westinghouse Electric Corporation, 1;;

Pittsburgh, Pa. 22 Filed: Mar. 5, 1971 1 ABSTRACT [211 App] N05 121,317 A turbogenerator set having an extraction control system for maintaining a selected pressure at each steam extraction point regardless of the flow at ing fi 'g dividual extraction points and irrespective of whether the load on the generatoris maintained at a selected Field of Search 73, load or whether the d is a as g as the load on the generator is greater than the power supplied by [56] References Cited that portion of the extracted steam which passes FOREIGN PATENTS OR APPLICATIONS through the turbine- 733,350 5/1966 Canada ..60/105 7 Claims, 3 Drawing Figures Q HP 6| 01L PATENTEDAPRS I975 3. 724,214

sum 1 [IF 2 STEAM do STEAM 6,

' I I I (I8 FIB i s I l 4 GENERATOR I I g l 13 I I5 C i l i CONDENSER 20 F PROCESS l 1 FIG.3

Q FACTORY ["1 9 LOAD TIE-LINE EXTRACTION CONTROL SYSTEM FOR A TURBOGENERATOR SET BACKGROUND OF THE INVENTION This invention relates to a control system for a turbogenerator set having extraction points for supplying various quantities of heating or process steam while carrying a set load on the generator. Presently employed extraction control systems substantially follow the same basic philosophy of load-compensation," i.e., schemes employing indirect open-loop systems that are predicated on the proposition that if each set of steam inlet control valves has a straight-line flow versus travel characteristic, it is possible to move these valves in proportional amounts to obtain steam flow conditions which are correctly proportioned to maintain a constant load on the turbine as the demands for extraction steam change.

Such schemes have two main objections, the first being the difficulty of providing valves having straightline flow versus travel characteristics, and the second being the proportional relationship of the valves change as the extraction pressure levels are altered, thus requiring skilled servicemen to reset the control system.

More particularly, load-compensation is inherently difficult to employ and inaccurate in operation, because process steam is extracted from a turbine after only partial expansion and hence does less work in carrying load than steam which passes through the entire turbine.

Accordingly, if steam is extracted, enough additional steam must be added to the turbine steam flow to balance the power loss due to extraction, otherwise the load is reduced. Theoretically, if the flow of extracted steam were to be measured, a proportionate measured the demand for extraction steam flow changed. The

above is uneconomical and unreliable, since as mentioned previously, it is very difficult to produce steam inlet control valves having the required characteristics, particularly when the extraction steam pressure values are changed, as required.

In addition to the above, extraction turbines carrying an electrical load, such as a generator, required to satisfy the customers electrical requirements, are often connected to large electrical utility tie-lines, so that additional power is provided in the event that greater electrical load is required by the customer than can be provided by the turbine. Under such installation conditions, the frequency of the generator output is determined by that of the utility tie-line.

In other instances, the turbine control system may not be effective to maintain its required selected load and the utility tie-line will be required to provide the balance.

SUMMARY OF THE INVENTION In general, a control system made in accordance with this invention controls a turbogenerator set comprising a first turbine unit having a first motive steam inlet control valve and an extraction steam outlet, a second turbine unit having a second motive steam inlet control valve provided with steam by the extraction steam outlet, and a load carried by the first and second turbine units. Such a control system maintains a selected extraction steam outlet pressure regardless of "the flow through the steam outlet and irrespective of the load on the generator and comprises means responsive to the speed of the turbine units for governing the inlet control valves, and means responsive to the steam pressure in the extraction steam outlet for modifying the governing effect on the first inlet control valve.

BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of this invention will become more apparent from reading the following detailed description in connection with the accompanying drawings, in which:

FIG. 1 diagrammatically shows a turbogenerator set having a control system made in accordance with this invention;

FIG. 2 diagrammatically shows a turbogenerator set having a hydraulic control system made in accordance with this invention;

FIG. 3 diagrammatically shows an electrical system and various inputs to a response device for a control system made in accordance with this invention.

PREFERRED EMBODIMENT Referring now to the drawings in detail, FIG. 1 diagrammatically shows a turbogenerator set comprising a high pressure or first turbine unit 1, an intermediate pressure or second turbine unit 2, and a low pressure or third turbine unit 3. The turbine units are mounted in tandem on a single shaft 4 with a generator 5. It is understood the turbine units may be in one casing or on separate shafts, the arrangement in FIG. 1 being shown for simplicity of explanation.

Steam from a boiler (not shown) flows through an inlet pipe 6, then through a first motive steam control valve 7, through the first turbine unit 1, and then through an extraction steam outlet or extraction point 9. A portion of the steam leaving the extraction steam outlet 9 is used for heating or for process steam, indicated by the box 10, and the remainder of the exhaust from turbine 1 flows through a second motive steam control valve 11, via pipe 12, to the intermediate turbine unit 2. After flowing through the intermediate tur bine 2, the steam flows through a second extraction steam outlet or extraction point 13. A portion of the steam from the second extraction outlet 13 is used for heating or process steam, indicated by the box 14, and the remainder of the steam flows through a third motive steam control valve 15, via pipe 16. The steam then flows through the low pressure turbine unit 3 into a condenser C.

FIG. 1 also shows a control system for maintaining a selected extraction steam pressure at each extraction point, regardless of the steam flow through the steam outlets and irrespective of whether the loading on the generator 5 is maintained at a selected load or whether the loading is varied. The control system comprises a device 17 responsive to the spead of the turbine units capable of producing a signal (electric, pneumatic or hydraulic) for governing the control valves 7, 11 and 15, causing them to be urged in closing direction as the speed increases above a preset value, and causing them to be urged in opening direction, if the speed decreases below this preset value. The signal from the speed responsive device is transmitted, via lines 18, to the various control valves.

Shown at 19 is a device responsive to the load on the generator capable of producing a signal for modifying the governing effect of the speed responsive device 17 on the control valves 7, 11 and 15. The load responsive device 19 is only made active when it is desirable to maintain a constant load on the generator or a load proportional to some reference load, i.e., the load of another generator or a system load. The load responsive device 19 is adapted to be made non-responsive to load or is disconnected from the control system if it is desired to allow the load on the generator to vary while maintaining the extraction steam pressures at their set values. The signal from the load responsive device is transmitted to the control valves 7, 11 and 15 via lines 20.

At 21 and 23, in FiG. 1, are shown devices responsive to the steam pressure of the extraction steam outlets 9 and 13, respectively. The pressure responsive devices 21 and 23 are capable of producing; a signal that modifies and opposes the governing effect of the speed responsive device and the load responsive device on the control valve 7 and 15, respectively. An. increase in pressure in the first extraction steam outlet 9 causes the pressure responsive device 21 to produce a signal which urges the first control valve 7 in closing direction and a pressure reduction in the first extraction steam outlet causes the opposite affect. An increase in the pressure in the second extraction outlet 13 causes the pressure responsive device 23 to produce a signal which urges the third control valve 15 in opening direction. A reduction in the second extraction pressure causes the opposite effect. Thus, it becomes apparent that the speed responsive and load responsive devices operate all the control valves to regulate the output of the turbogenerator while the pressure responsive devices 21 and 23 operate control valves 7 and 15, respectively, through lines 24 and 25 to regulate the pressures of the extraction steam outlets. Thus, even through the control valves do not have straight line flow versus travel characteristics, the steam pressure at the extraction points will be maintained constant as the load set on the generator is varied or as the: requirements for process steam vary.

The operation of the control system can best be understood by following the action of the various components during a normal operating cycle. Assume a preset load on the generator and a selected pressure for the process steam to the processes indicated by the boxes and 14, which would provide a specific pressure in the extraction outlets 9 and 13. With the load responsive device 19 set to maintain a selected load, a decrease in the demand for steam to process 10 or to process 14, or to both, causes additional steam to pass through the turbine units 3 or turbine units 2 and 3, which would tend to make the turbogenerator set speed up. The speed responsive device 17 would sense the increase in speed and send a signal to control valves 7, 1 1- and 15 causing each valve to tend to close. If the generator 5 were connected to an electrical system with several other turbogenerator units, the increase in speed would cause an increase in load. The load responsive device sensing this load increase would send a modifying signal to control valves 7, 11 and 15 cansing them to close further. If the process 10 by itself were to have the decrease in demand (increase in pressure at the extraction outlet 9), the control valve 7 would also receive a signal from the pressure responsive device 21 causing it to be urged in closing direction, thus it would close to a greater extent than control valves 11 and 15.

To rebalance the system for the new flow conditions requiring less steam for process 10, it becomes necessary to close control valve 7 more than control valves 1 1 and 15. If the demand for steam for process 14 were to decrease, the speed and load responsive devices 17 and 19 would sense an increase in speed and load due to the additional steam flowing through the low pressure turbine unit 3 and send signals to the control valves 7, 11 and 15 urging them in closing direction. The reduction in flow to the process 14 (increase in the pressure in the extraction outlet 13) would cause the pressure responsive device 23 to send a signal to control valve 15 modifying and opposing the signal from the load and speed responsive device causing control valve 15 not to close as much as the other control valves 7 and 11, or if the pressure change were large enough, the signal from the pressure responsive device 23 could cause the control valve 15 to move in opening direction. A reduction in the demand of steam forprocess 14 necessitates the smaller flow of steam through turbine units 1 and 2 and the greater flow through turbine unit 3, to maintain the same load on the generaton'which this control system provides.

A decrease in the flow requirements of both processes 10 and 14 (pressure at extraction outlets 9 and 13 increase) would cause the turbine speed and load to increase. The speed and load responsive devices 17 and 19 sensing these changes would send signals to urge control valves 7, l1 and 15 in closing direction, the pressure responsive device 21 would modify the signal to control valve 7 causing it to close a greater amount, and the pressure responsive device 23 would modify the signal to control valve 15 to cause it to close less or even to move in opening direction, depending on the degree of reduction in the flow of steam to process 14. The new flow conditions required for reduction of flow to processes 10 and 14 would necessitate a reduction in the steam flow to the high pressure turbine, a slightly smaller reduction in the steam flow to the intermediate pressure turbine, and a very slight reduction or an increase in the flow of steam to the low pressure turbine, which this control system provides.

it has been noted that the pressure responsive devices move only one control valve, while the load and speed responsive devices cause simultaneous action on all control valves to hold the load at its selected value. Load variations follow steam flow changes with very small time delay, whereas the change in pressure in the extraction piping system has a long time constant, because of the large volume of steam normally in such systems. Thus, to obtain close steady state control, a long time constant must be used to compensate for the flow changes in the extraction outlets. The speed and load responsive devices trim the steam flow as the pressure responsive devices return the pressure to its selected value, providing precise steam pressure control at the extraction points, although the valves do not have straight line flow versus travel characteristics.

As noted earlier, the sensing devices can produce electrical, hydraulic or pneumatic signals, therefore it is understood that the control system of this invention may use electrical, hydraulic or pneumatic media or any combination thereof for controlling the turbogenerator set.

FIG. 2 diagrammatically shows a control system of this invention using hydraulic controls. As noted, it is understood that pneumatic, electrical, or any combination thereof, may also be employed. As in FIG. 1, the turbogenerator set in FIG. 2 comprises the high pressure turbine 1, the intermediate turbine 2., the low pressure turbine 3, and the generator 5 mounted on a single shaft 4. Motive steam from the boiler (not shown) flows through the inlet piping 6, the first motive steam control valve 7, and then through the high pressure turbine 1. The motive steam is exhausted from turbine 1 through the extraction outlet 9. A portion of the steam leaving the extraction outlet 9 is used for heating or process steam, indicated by the box 10. The remainder of the exhaust from turbine 1 flows through a second motive steam valve 11 via pipe 12 before entering the intermediate turbine 2. The exhaust from turbine 2 flows through the extraction outlet 13 and then divides. A portion of the steam is utilized for heating or process steam, indicated by box 14, and the remainder of the steam flows through the third motive control valve 15 via line 16, through the low pressure turbine 3 and then into the condenser C.

FIG. 2 also shows a hydraulic control system for maintaining selected extraction pressures at the extraction points, regardless of the steam flow, when the load on the generator is maintained at a selected value.

The hydraulic control system, as shown in FIG. 2, comprises three servomotors or hydraulic cylinders 31, 32 and 33 which operate the control valves 7, 11 and 15, respectively. Each servomotor has a piston 34, 35 and 36 which divides the servomotor into upper and lower chambers 37, 38 and 39 and 40, 41 and 42, respectively. if the pressure in the upper chambers 37, 38 and 39 exceeds the pressure in the lower chambers 40, 41 and 42 the valves will open and vice versa. The lower chambers 40, 41 and 42 of the servomotors 31, 32 and 33 are in communication with a hydraulic conduit 45. The conduit 45 is in communication with a supply of high pressure hydraulic fluid 46, the flow of the fluid from the supply being controlled by restrictions 47 and 48 disposed in a cover plate 49 of a reservoir 50. Check valves 51 and 53 are disposed adjacent each end of the conduit 45 downstream of the restrictions.

The conduit 45 is in communication with the hydraulic reservoir 50. Controlling the flow of hydraulic fluid from the conduit 45 to the reservoir 50 are cup valves 57 and 59, which seat on the cover plate 49. Cup valve 57 is controlled by the speed responsive device 17 which in FIG. 2 is shown to comprise an impeller 61,

mounted on the turbine shaft 4 to produce a hydraulic pressure which increases as the square of the speed. Hydraulic conduit 63 transmits the pressure produced by the impeller 61 to a bellows 65 disposed in a cavity 66 in the cover plate 49. A balance bar 67, pivotally mounted at its midpoint to the cover plate 49 by a mounting bracket 68, controls the movement of the cup valve 57, which is cooperatively associated with one end thereof. The other end of the balance bar 67 is biased in a downward direction by pressure in conduit 63 acting on the bellow 65 and biased in an upward direction by a compression spring 69 mounted in the reservoir 50. The bias of the spring 69 may be changed manually by the hand wheel 71 mounted outside of the reservoir 50 for turning a screw 72 extending into the reservoir to increase the compression on the spring 69. The bellows 65, spring 69 and cup valve 57 are so disposed relative to the balance bar 67, that an increase in speed of the turbine and governing impeller 61 causes an increase in pressure in line 63, resulting in cup valve 57 moving in closing direction and an increase in the pressure in the conduit 45 which causes the pistons 34, 35 and 36 of the servomotors 31, 32 and 33 to move upwardly, urging the control valves 7, l1 and IS in closing direction to return the turbine to its set speed. A reduction in speed causes a reduction in pressure in conduit 63 and causes the servomotors 31, 32 and 33 to urge the control valves 7, l1 and 15 in opening direction.

The load responsive device 19 is shown in FIG. 2 to comprise a transducer 73, which controls an electrical motor 74 having a torque arm 75 extending through the side of the reservoir 50. The torque arm 75 is biased in a counterclockwise direction by a tension spring 77 disposed in the reservoir. The bias of the spring 77 is regulated by a hand wheel 79 mounted outside the reservoir 50 for turning a screw 80 extending into the reservoir to increase or decrease the tension on the spring 77. The cup valve 59 is cooperatively associated with the torque arm 75 and moves in closing direction toward the cover plate 49 when the torque arm 75 moves in clockwise direction. Upon sensing an increase in load, the load responsive device sends a signal to the motor 73 via lines 81 causing the motor 74 to rotate the torque arm in clockwise direction operating the cup valves 59 in closing direction to increase the pressure in the conduit 45. This causes the pistons 34, 35 and 36 to move in an upward direction to urge the control valves 7, 11 and 15 to move in closing direction. A bleed conduit 83 connects conduit 45 to reservoir 50 and a restriction 85 controls the rate of flow through conduit 83. The check valves 51 and 53 and bleed conduit 83 cooperate to allow either impeller 61 and its associated controls responsive to the turbine speed or transducer 73 and its associated controls responsive to the generator load to override the other when one of these controllers produces a higher pressure in conduit 45 than the other. Thus, it is possible to set the load controller to some minimum load, presumably higher than the power produced by the required process steam as it passes through the turbine units, and allow the speed responsive controller to control the turbogenerator unit as the generator load varies above this preset minimum level.

As shown in FIG. 2, the servomotor 31 has its upper chamber 37 in communication with hydraulic conduit 87, which has the flow from one end thereof regulated by cup valve 89, which seats against the cover plate 49. Conduit 87 is in communication with the supply of high pressure hydraulic fluid 46 disposed in the cover plate 49. The flow of hydraulic fluid from the supply 46 to conduit 87 is regulated by a restriction 90, also disposed in the cover plate 49. A bellows 91 is disposed in a cavity 66' in the cover plate in communication with the extraction outlet 9 via conduit 92. A link 93 is pivotally mounted at one of its ends on a mounting bracket 68' fastened to the cover plate 49 and is biased downwardly by a pressure acting on the bellows 91 and is biased upwardly by a compression spring 94 disposed within the reservoir 50. The upward bias of the spring 94 may be increased or decreased by turning the hand wheel 95 mounted outside the reservoir 50 which operates a screw 96 extending into the reservoir to increase and decrease the compression on the spring 94. The cup valve 89 is cooperatively associated with the link 93 in such a manner, that a decrease in pressure at the exhaust point 9 and in conduit 92 causes the cup valve to move in closing direction, toward the cover plate 49, resulting in an increase in pressure in conduit 87 and in the upper chamber 37 of the servomotor, 31. Thus, causing the control valve 7 to be urged in opening direction, resulting in greater steam flow to the high pressure turbine and an increase in the pressure at the extraction outlet 9.

An increase in the pressure at the extraction outlet 9 causes the control valve 7 to move in the opposite direction, that is, in a closing direction. This, of course, would cause a decrease in the pressure at the extraction point 9. The upper chamber 38 of the servomotor 32 is in communication with the high pressure hydraulic fluid supply via conduit 97 and restrictions 99 and 100 fix the pressure in the upward chamber 38 so that the servomotor 32 is only responsive to changes in the pressure in conduit 45, which is regulated by the cup valves 57 and 59 which in turn are part of the speed and load responsive devices 17 and 19, respectively. The upper chamber 39 of the servomotor 33 is in communication with a conduit 101. The conduit 101 is supplied with pressurized hydraulic fluid from the supply 46 through a restriction 103, disposed within the cover plate 49, and a cup valve 105 which seats on the cover plate 49 regulates the pressure within the conduit 101. A balance arm 107 is pivotally mounted at its midpoint on a mounting bracket 68" and has one end biased in downward direction by the pressure of the second extraction outlet 13 acting via conduit 108 on a bellows 109 disposed in a cavity 66" in the cover plate 49. A compression spring 113 exerts an upward bias on the one end of the balance arm 107 to counteract the bias in the downward direction exerted by the bellows 109. A hand wheel 115 disposed outside the reservoir 50 is used to turn a screw 116 which extends into the reservoir to increase or decrease the bias of the spring 113. The cup valve 105 is cooperatively associated with the balance arm 107 in such a manner that an increase in pressure at the extraction outlet 13 is transmitted to the bellows 109, via conduit 108, causing the cup valve 105 to move in closing direction, resulting in an increase in pressure in the conduit 101 and in the upper chamber 39 of the servomotor 33. This causes the piston 36 to move downwardly and the control valve 15 to be urged in opening direction. This in turn reduces the pressure of the extraction outlet 13.

The lower chambers 40, 41 and 42 of the servomotors 31, 32 and 33 are subject to the control of either the speed or load responsive devices or a combination thereof to change the flow of steam to all three turbine units "upon a change in speed of the shaft 'or a change in the load on the generator. The upper chambers 37 and 39 are subject to changes in the extraction outlet pressures to urge the control valves 7 and 15 in opening or closing directions to maintain the extraction outlets at their set pressure by opposing or modifying the affects of changes in pressure in the lower chambers 40, 41 and 42. The hydraulic control system, as shown in FIG. 2 and herebefore described, among other functions will maintain the extraction points at a set pressure even with large variation in load, although the control valves do not have straight line flow versus travel characteristics.

As shown in FIG. 3, the load responsive device can also be made responsive to the load on another generator 117, an electrical tie line as indicated at 119 or a factory load 121 to proportion the output of the generator 5 relative to the output of the other generators, a system load, or factory load.

Transducers 73, 123 and 125, as shown in FIG. 3, respond to the controlled turbogenerators load 5;, the load on another generator 117 or the factory load 121, respectively, to transmit a signal via lines 131, 133 and 135, respectively, to a selector 137. The selector 137 receives the signal from the various transducers, integrates them, and sends a signal via line 139 to the motor 74, which operates the cup valve 59 to urge the control valves 7, 11 and 15 in opening or closing directions (see FIG. 2). The selector may be set so that the signal from the transducer 73 responsive to the load on the controlled generator 5 is transmitted directly to the motor 74 to urge the control valve in opening or closing directions to maintain a set load on the generator 5. If desired, the selector may be set to receive signals from the controlled generators transducer 73 as well as from the other generators transducer 123 via lines 131 and 133, to integrate the signal and to transmit a signal to the motor 74 via line 139 causing the motor to operate the cup valve 59 to urge the control valve in opening or closing directions for maintaining a load on the controlled generator 5 proportional to the load on the other generator 117.

In a similar manner the selector may be set to receive signals from the factory loads transducer and the controlled generators transducer 73, integrate the signals and transmit the integrated signals-to the motor 74 to cause the control valves to be urged in opening or closing directions to maintain a load on the control generator 5 proportional to the factory load.

The load on the controlled generator may be made proportional to a system load, shown in FIG. 3 to comprise the output of the generators 5 and 117, the input or output of the tie line 119, and the factory load 121. The transducers 73, 123 and 125 each send signals to the selector 137 and the selector integrates the signals and sends a signal via line 139 to the motor 74 to cause control valves 7, 11 and 15 to be urged in opening or closing directions to maintain the load on the generator proportional to the system load. Thus, the hydraulic control systems shown in FIG. 2 combined with the load responsive devices shown in FIG. 3 produce a versatile control system for turbogenerator sets having one or more extraction points.

What is claimed is:

1. In a turbogenerator set comprising a first turbine unit having a first motive steam inlet control valve and an extraction steam outlet, a second turbine unit having a second motive steam inlet control valve provided with steam by said extraction steam outlet, said extraction steam outlet also being adapted to supply steam to a process having varying steam demands, and a load carried by said first and second turbine units;

a control system for maintaining a set speed on said turbines and supplying said varying demands for steam of said process comprising means responsive to the speed of the turbine unit for jointly governing said first and second inlet control valves, and

means responsive to the steam pressure in said extraction steam outlet for opposing and modifying the governing effect of said speed responsive means on said first inlet control valve to supply the varying demands of steam of said process and maintaining a set speed on said turbines.

2. A control system as set forth in claim 1 and further comprising means responsive to load for modifying the governing effect of the speed responsive means jointly on the first and second inlet control valves and being capable of maintaining said load at a selected value, and being capable of supplying the varying demands for steam of the process.

3. A control system as set forth in claim 1 and further comprising means responsive to a system load for modifying the governing effect of the speed responsive means to proportion the load on the turbines relative to said system load, and being capable of supplying the varying demands for steam of the process.

4. In a turbogenerator set as set forth in claim 1, wherein the second turbine unit has a second extraction steam outlet, and further comprises a third turbine unit having a third motive steam inlet control valve provided with steam from said second extraction steam outlet, said second extraction steam outlet also being adapted to supply varying demands for steam ofa second process;

the control system having means responsive to the speed of said turbine units for governing all the inlet control valves and means responsive to the steam pressure, in said second extraction steam outlet, for modifying and opposing the governing effect of said speed responsive means on said third inlet control valve, to supply the varying demands second extraction steam outlet, said second extraction steam outlet also being adapted to supply varying demands for steam of a second process;

the control system having means responsive to the speed of the turbine units for governing all the inlet control valves;

means responsive to load for modifying the governing effect of the speed responsive means on all said inlet control valves, and being capable of maintaining the load at a selected value, and

means responsive to the steam pressure in said second extraction steam outlet for modifying and opposing said modified governing effect of the speed and load responsive means on said third inlet control valve to supply the varying demands for steam of said second process.

6. In a turbogenerator set as set forth in claim 1,

wherein the second turbine unit has a secondextracting steam outlet supplying varying demands for steam of a second process and further comprises,

a third turbine unit having a third motive steam inlet control valve provided with steam from said second extraction steam outlet,

the control system having means responsive to the speed of the turbine units for governing all the inlet control valves,

means responsive to a system load for modifying the governing effect of the speed responsive means on all the inlet control valves to proportion the load on the turbine units relative to a system load, and

means responsive to the steam pressure in said second extraction outlet for modifying and opposing said modified governing effect of the speed and system load responsive means on said third inlet control valve to supply the varying demands for steam of said second process.

7. In a steam turbogenerator set comprising a high pressure turbine unit having a first extraction steam outlet, which supplies varying demands for steam of a first process,

an intermediate turbine unit provided with motive steam by said first outlet and having a second extraction steam outlet, which supplies varying demands for steam of a second process,

a low pressure turbine unit provided with motive steam by said second outlet, and

a load carried by said turbine units;

a control system for maintaining said load at a selected value with varying demand for steam at selected pressure levels for said processes, comprising a first steam inlet control valve for regulating flow of high pressure motive steam to said high pressure unit,

a second steam inlet control valve interposed between said first extraction outlet and said intermediate pressure turbine unit for regulating flow of intermediate pressure motive steam to the latter unit,

a third steam inlet control valve interposed between said second extraction steam outlet and said low pressure turbine unit for regulating flow of low pressure motive steam to the low pressure turbine unit,

opening direction with decreasing extraction steam pressure, and

means responsive to steam pressure in said second extraction steam outlet and providing a second extraction pressure signal effective to oppose said modified signal and urge said third control valve in opening direction with increasing extraction steam pressure in opposition to the effect of said modified signal.

a: a: a 

1. In a turbogenerator set comprising a first turbine unit having a first motive steam inlet control valve and an extraction steam outlet, a second turbine unit having a second motive steam inlet control valve provided with steam by said extraction steam outlet, said extraction steam outlet also being adapted to supply steam to a process having varying steam demands, and a load carried by said first and second turbine units; a control system for maintaining a set speed on said turbines and supplying said varying demands for steam of said process comprising means responsive to the speed of the turbine unit for jointly governing said first and second inlet control valves, and means responsive to the steam pressure in said extraction steam outlet for opposing and modifying the governing effect of said speed responsive means on said first inlet control valve to supply the varying demands of steam of said process and maintaining a set speed on said turbines.
 2. A control system as set forth in claim 1 and further comprising means responsive to load for modifying the governing effect of the speed responsive means jointly on the first and second inlet control valves and being capable of maintaining said load at a selected value, and being capable of supplying the varying demands for steam of the process.
 3. A control system as set forth in claim 1 and further comprising means responsive to a system load for modifying the governing effect of the speed responsive means to proportion the load on the turbines relative to said system load, and being capable of supplying the varying demands for steam of the process.
 4. In a turbogenerator set as set forth in claim 1, wherein the second turbine unit has a second extraction steam outlet, and further comprises a third turbine unit having a third motive steam inlet control valve provided with steam from said second extraction steam outlet, said second extraction steam outlet also being adapted to supply varying demands for steam of a second process; the control system having means responsive to the speed of said turbine units for governing all the inlet control valves and means responsive to the steam pressure, in said second extraction steam outlet, for modifying and opposing the governing effect of said speed responsive means on said third inlet control valve, to supply the varying demands for steam of said second process.
 5. In a turbogenerator set as set forth in claim 1, wherein the second turbine unit has a second extraction steam outlet, and further comprises a third turbine unit having a third motive steam inlet control valve provided with steam from said second extraction steam outlet, said second extraction steam outlet also being adapted to supply varying demands for steam of a second process; the control system having means responsive to the speed of the turbine units for governing all the inlet control valves; means responsive to load for modifying the governing effect of the speed responsive means on all said inlet control valves, and being capable of maintaining the load at a selected value, and means responsive to the steam pressure in said second extraction steam outlet for modifying and opposing said modified governing effect of the speed and load responsive means on said third inlet control valve to supply the varying demands for steam of said second process.
 6. In a turbogenerator set as set forth in claim 1, wherein the second turbine unit has a second extracting steam outlet supplying varying demands for steam of a second process and further comprises, a third turbine unit having a third motive steam inlet control valve provided with steam from said second extraction steam outlet, the control system having means responsive to the speed of the turbine units for governing all the inlet control valves, means responsive to a system load for modifying the governing effect of the speed responsive means on all the inlet control valves to proportion the load on the turbine units relative to a system load, and means responsive to the steam pressure in said second extraction outlet for modifying and opposing said modified governing effect of the speed and system load responsive means on said third inlet control valve to supply the varying demands for steam of said second process.
 7. In a steam turbogenerator set comprising a high pressure turbine unit having a first extraction steam outlet, which supplies varying demands for steam of a first process, an intermediate turbine unit provided with motive steam by said first outlet and having a second extraction steam outlet, which supplies varying demands for steam of a second process, a low pressure turbine unit provided with motive steam by said second outlet, and a load carried by said turbine units; a control system for maintaining said load at a selected value with varying demand for steam at selected pressure levels for said processes, comprising a first steam inlet control valve for regulating flow of high pressure motive steam to said high pressure unit, a second steam inlet control valve interposed between said first extraction outlet and said intermediate pressure turbine unit for regulating flow of intermediate pressure motive steam to the latter unit, a third steam inlet control valve interposed between said second extraction steam outlet and said low pressure turbine unit for regulating flow of low pressure motive steam to the low pressure turbine unit, means responsive to speed of said turbine units and providing a speed signal effective to urge each of said control valves in closing direction with increasing speed, means responsive to said load and providing a load signal modifying said speed signal and effective to augment the speed signal with increasing load, means responsive to steam pressure in said first extraction steam outlet and providing a first extraction pressure signal effective to oppose said modified signal and urge said first control valve in opening direction with decreasing extraction steam pressure, and means responsive to steam pressure in said second extraction steam outlet and providing a second extraction pressure signal effective to oppose said modified signal and urge said third control valve in opening direction with increasing extraction steam pressure in opposition to the effect of said modified signal. 